The United States or other countries will one day be forced to consider pre-emptive action if Iran and North Korea continue to seek nuclear weapons, a senior U.S. government official said on Tuesday.

The United States and its allies have accused Iran of pursuing nuclear weapons under the guise of a civilian energy program and are pushing for United Nations' sanctions. Tehran denies the accusation.

North Korea conducted an underground test of what was believed to have been a small nuclear weapon last month.

If North Korea refused to renounce its nuclear program and Iran developed a nuclear weapons capability, it would lead other countries in their regions to seek nuclear weapons, said the U.S. official, speaking on condition he was not identified.

"We, the United States, and others who might be threatened by these developments will have to look at how to respond and inevitably I think people will have to look at the question of pre-emption," the official told reporters.

"I think it's inevitable that any American administration, not just this administration but future administrations, will have to look at pre-emptive strategies," he said.

He was not saying that the United States was going to launch a pre-emptive strike "tomorrow", he said.

He said there could be uncertainty, for example, over whether a missile in the hands of one of the countries had a nuclear warhead attached.

"Under those circumstances some people might be inclined to pre-emption, and not just the United States ... Others who might feel threatened in the region might feel more inclined to pre-emption as a defense," he said.

"It's a challenge for the international community." He urged the world to think hard about the cost of allowing Iran to "continue to move down the road it's been on".

President Bush says he backs a diplomatic solution in the case of Iran but has refused to rule out a military strike.

A U.S. attack against Iran would set back Tehran's suspected quest for nuclear arms by less than four years, U.S. officials and independent experts say.

Some analysts have speculated Israel could consider a strike against Iran if it felt threatened. An Iranian official said this week Iran would respond swiftly if Israel attacked it

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